1. Field of the Invention
The present invention relates to heterocyclic compounds and, more specifically, to fused ring heterocyclic compounds and organic light-emitting devices including the same. Additionally, the present invention relates to organic light-emitting devices including the subject heterocyclic compounds and having improved light emitting properties.
2. Description of the Related Art
Organic light-emitting devices (OLEDs), which are self-emitting devices, have advantages such as wide viewing angles, excellent contrast, quick response, high brightness and excellent driving voltage characteristics and can provide multicolored images. Due to these characteristics, OLEDs have been receiving a growing level of attention.
An existing organic light-emitting device has a structure that includes an anode disposed on a substrate, a hole transport layer (HTL), an emission layer (EML), an electron transport layer (ETL) and a cathode that are sequentially disposed upon one another. The HTL, the EML, and the ETL are normally formed of organic compounds. When a voltage is applied between the anode and the cathode, holes injected from the anode move to the EML via the HTL, and electrons injected from the cathode move to the EML via the ETL. The holes and electrons (carriers) recombine in the EML to generate excitons. When the exitons drop from an excited state to a ground state, light is emitted.
A major factor affecting luminescent efficiency of an organic light-emitting device is the structure of the organic luminescent material used to construct the device. Fluorescent materials have been widely used as organic light-emitting materials, but they may have limited emission efficiency due to having a mere 25% probability of being in an excited singlet state. Meanwhile, phosphorescent materials have a 25% probability of being in an excited singlet state and a 75% of probability of being in an excited triplet state, and thus they may improve emission efficiency up to four times as compared with fluorescent materials, based on theoretical electroluminescence mechanisms, and they may achieve a 100% internal quantum efficiency. Thus, the use of phosphorescent materials in organic light-emitting devices is gradually increasing.
Anthracene derivatives are typical organic light-emitting materials. However, an organic light-emitting device using an anthracene derivative in which two or three anthracene moieties are linked in a conjugated system may have a narrow energy gap, a low blue-light color purity, and a low emission efficiency. Thus, improving upon the performance obtainable with anthracenes is desirable.
4,4′-Bis(carbazole-9-yl)biphenyl (CBP) is in wide use as a phosphorescent host material, but it is not suitable for green-light emission due to having a wide bandgap and reduced emission efficiency. It is also not easy to control hole or electron mobility with this material to adjust the charge balance.
These existing organic light-emitting materials are not satisfactory in emission characteristics, including emission efficiency, and thus there is still a demand for improvement.